In recent years, there have been proposed and developed various electric-motor-driven valve timing control devices in which rotary motion (a torque) of an electric motor is transmitted via a speed reducer to a camshaft so as to change a relative angular phase between the engine crankshaft and the camshaft with the high control responsiveness and high controllability. One such electric-motor-driven valve timing control device has been disclosed in Japanese Patent Provisional Publication No. 2010-138735 (hereinafter is referred to as “JP2010-138735”). In the valve timing control device disclosed in JP2010-138735, by virtue of electric-current supply via spring-loaded brushes and slip rings to an electric motor, the motor is rotated. The rotary motion of the electric motor is transmitted via a speed reducer to a camshaft, and as a result an angular phase of the camshaft relative to the crankshaft is changed to control engine valve timing, such as intake valve timing.
However, the valve timing control device disclosed in JP2010-138735, suffers from the drawback that, when initiating relative-phase control between the crankshaft and the camshaft during an engine starting period, in particular, when starting with a cold engine, an electric motor is driven from its stopped state and thus a time loss occurs owing to a static friction before the electric motor actually begins to rotate and hence undesirable hunting of the automatic phase control system occurs. As a result of such undesirable hunting, a control state of the phase control system tends to become unstable immediately after the electric motor has been driven. Therefore, it would be desirable to reconcile both a phase-change control responsiveness and a phase-change control stability without undesirable hunting, even during an engine starting period.